In the medical diagnostic field, transducers are typically arranged into arrays for use in an ultrasonic imaging system. For example, FIG. 1 shows an ultrasonic imaging system 100 in which an electrical stimulus 102, such as a pulse, is provided to an array of transducers 101, causing the transducers to transmit an ultrasonic acoustical wave 103. The ultrasonic wave is transmitted into the human body and eventually at least partially reflected by an object in the body, for example the heart 115. The reflected wave ("echo") is received by the transducer array 101 which produces an electrical current 104 that is indicative of the echo. Various properties of signal 104, such as its amplitude and phase, are then analyzed by signal processing to determine information about the object, such as its size, location, and velocity. See for example U.S. Pat. No. 5,060,651 entitled "Ultrasonic Diagnostic Apparatus."
More specifically, FIG. 1 shows a microprocessor 108 for controlling each of a transmitter 107, preamplifier 109, beam former 106, and digital scan converter 111. The echo signal 104 from transducer array 101 is sent to the preamplifier 109 and then in series to beam former 106, signal processor 105, A/D converter 110, and digital scan converter 111. The z-component of the echo signal is sent to post processor 112, and the resulting z-intensity is displayed on CRT screen 114. The x-y component is sent via x-y raster 113 and displayed on CRT screen 114. Any number of different transmitting and imaging processing system may be used.
Acoustic imaging transducers are generally made of piezoelectric materials, such as lead zirconate titanate (PZT) ceramic. They may also be formed of "composites", in which a piezoelectric ceramic and a polymer are combined for an improved range of properties.
Acoustic imaging transducers have also been made of electrostrictive materials, which are highly polarizable by the application of a DC bias voltage. Lead magnesium niobate-lead titanate (PMN-PT) is one example of an electrostrictive ceramic.
Both piezoelectric and electrostrictive transducer elements may be characterized by their pulse/echo response sensitivity. "Sensitivity," generally refers to how efficiently energy is transmitted (output) for a given input. "Pulse/echo response" refers to the transducer's capability of converting electrical energy into acoustical energy, and emitted acoustic energy back to electrical energy; it thus combines both the transmit and receive efficiencies. It is defined by the electrical pulse stimulus as an input, and acoustical energy as an output.
The pulse/echo response sensitivity of an electrostrictive transducer depends on various material properties, such as the relative dielectric constant K and the coupling coefficient k.sub.t. These properties are a non-linear function of the operating temperature, the DC bias voltage, and the frequency of operation. See, for example Takeuchi et al., "Relaxor Ferroelectric Transducers," Central Research Laboratory, Hitachi, Ltd. (1991). In addition, these properties may vary because of inherent variances in the starting material and/or the processing conditions. As such, the sensitivity may fluctuate for various reasons, some of which may be difficult to control.
The Federal Drug Administration (FDA) regulates the amount of acoustic energy that may be transmitted into a human body. If the sensitivity of a transducer varies, an excessive amount of energy may be transmitted. Moreover, systems which allow their input/output relationship to vary substantially are often perceived in the marketplace as being of inferior quality.
To date, the prior art has driven acoustic piezoelectric transducers with an open control loop. Thus, the prior art system would electrically stimulate the transducer, but not monitor the sensitivity to detect variations in sensitivity.
In summary, there is a need for apparatus and methods for controlling the sensitivity of transducer elements.